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need LARGE light meter

Discussion in 'Lighting' started by Jim, Jan 31, 2004.

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  1. Jim

    Jim Guest

    It was suggested by someone in, where this was
    originally posted, that I also post this here. so here goes;

    I need to build a light meter that will average the intensity if visible
    light over a large area.....possibly a square foot or so.

    I have a number of light meters that use what I think is a "blue enhanced"
    sensor behind a white plastic translucent filter of about 2 inches or so in
    diameter. As far as I can tell the main purpose of this white filter is to
    provide an average of the light intensity that may be hitting different
    parts of it's surface. Since the sensors in all of these I own has an
    active area of around 2.5 x 2.5 mm and the sensor is located below and in
    the center of this white filter, I guess that the way it works is that the
    sensor "see's" the whole of the bottom surface of the white disk and
    responds to the diffuse light coming through, bouncing around, and whatever.

    What I need though is the equivelent that can do the same averaging trick
    over a much larger area. So far I have made a large diffuse filter by
    cutting a frosted white glass light fixture globe so that it is a true
    hemi-sphere. I think the diameter is around 10 inches. It's frosted on the
    inside, but I couldn't find one frosted on the outside which I would have
    preffered. Unless I find something better, this will be my diffuse filter.

    I am wondering about sensors now. I have a couple of types of blue enhanced
    sensors which have a voltage output, which are suppose to somewhat mimic the
    spectral response of the human eye. This is a bell shaped curve from
    approximately 400 to 700 nm. Two questions on this; first, since the
    primary purpose of this light meter it to measure the amount of sunlight
    that is reaching the plants on a forest floor (the light filtering through
    the canopy above), I would prefer that the spectral response of my sensor
    to be much more flat from 400 to 700 nm....., the photosynthesis active
    radiation there a sensor that is available that can do this?
    How about adding glass filters to a blue enhanced filter?

    Secondly, does one sensor mounted in the center really do the trick in
    "seeing" the whole of the inside of a hemi-sphere this size, or do I need to
    consider an additional circular "array" of 5 or 6 sensors mounted flat
    around the center one. If I do this with voltage output sensors, how do I
    connect them together? Does wiring them in parallel provide an "average"
    output of what all are sensing?

    I can buy a light meter on ebay or somewhere that I can salvage the
    electronics and the display from, and call the displayed readings "relative"
    and either create a lookup table or calibrate it with another light meter
    somehow, but for now this is a separate problem.

    Can someone give some ideas or suggections on the sensor issues and any
    other issues?

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